"Transformative" is exactly what they're worried about.

So take Cervantes and bring him back to life in this century or the last. Make him attend a performance of Man of la Mancha. More to the point, make him listen to the theater-goers gushing over the beauty of dreaming the impossible dream.

So take Machiavelli into the modern day and let him look up his name in the English dictionary. Then ask him to finally tell us what he was really thinking when he wrote The Prince and if maybe we should've read a little more of his works before making his name a byword for deception.

And then say that in 2013, someone writes a His Dark Materials fanfiction about the creation of the Church of the Dust, and a hundred years later, a post-apocalyptic society constructs this Church and sends its Silvertonguist preachers out to spread the word. How about that, Philip Pullman?

Here's the thing. In fandom we often perceive the debate over the legitimacy of fanfiction as motivated by angry, possessive authors seized by ridiculous feelings of attachment to their characters, unable to understand that fanfiction is not actually a profound sin akin to adultery. Either that, or otherwise sympathetic authors haunted by the legal ghost of Marion Zimmer Bradley, fearful of litigation from an unscrupulous fan. Probably because those actually are the loudest voices.

But not everyone crying, "Intellectual property," is conjuring an ethical chimera or bowing to their fear of the law's misuse. Some of them are voicing a very real conceptual concern, hinted at when they start to talk about trademark and copyright laws intended to prevent someone from passing off a false product as that of the original author's. They are not making an irrelevant and specious comparison. (Some of the time, anyway.) They mean exactly what they say: they fear that fanfiction, unrestrained, could take the pure and true story they have given the world and change it in the eyes of the public. Because sometimes it actually can.

Here's an example fandom might be more interested in: where do you think certain Harry Potter fans got the idea that Blaise Zabini had to be white? Okay, yes, racism. But fanfiction probably helped convey it.

When some professional creators of fiction argue against fanfiction, what they are saying is not, "It makes me uncomfortable, those characters belong to me and it's wrong for you to play with them." Instead, it is, "I have defined those characters, and I would like to exercise my right to keep that definition from changing."

They're still wrong. They have been told repeatedly, often by other professional creators of fiction, why they are wrong. But this is a different (if related, and not entirely distinct) argument in nature than "fanfiction is disgusting and wrong and violates my ownership of these people who are like children or spouses to me" versus "seriously, they're just characters, nobody is trying to have an affair with your spouse." It's actually an argument between the modern idea of "intellectual property" as a defensible ethical concept and the human reality of the ancient and eternal shared and fickle nature of culture.

There are also nits to pick, and complexities undefined. As Niccolò up there would point out, it's not just straight-up fanfiction that transforms, but also shared commentary and interpretation, and nobody could ever ban those. Not to mention: virtually everyone will concede that the dead can no longer protect their intellectual property...eventually. Maybe their children and grandchildren get to do it for a while, and where do you stop? Whatever: Anglo-Saxon #74839 in the year eight-hundred-and-yonk can hand it over to John Gardner by the time the 1970s roll around, but goddammit, my kids are going to make sure nobody lets women with curly hair become spirit-bonded with space otters for another fifty years.

Anyway, not everyone's looking at it from the perspective of the ones comparing fanfiction authors to adulterers and kidnappers. Some are coming at it from the perspective of an angry Spanish satirist who thinks the music's terrible, an Italian republican who wishes we'd remember that he really hated the Medicis, or Android Pullman in 2287 fighting the new theocratic forces of Lyra's Disciples from the haven of the Brights' Satellite.